Secondary emission control in an orthicon camera tube



May 26, 1964 E. D. HEN DRY SECONDARY EMISSION CONTROL IN AN ORTHICON CAMERA TUBE Filed May 24, 1962 I NV ENTQR United States Patent .0

3,134,926 SECONDARY EMISSION CONTROL IN AN ORTHICON CAMERA TUBE Eric Douglas Hendr'y, Essex, England, assignor to English Electric Valve Company Limited, London, England, a

British company Filed May 24, 1962, Ser. No. 197,427 Claims priority, application Great Britain Apr. 13, 1962 4 Claims. (Cl. 315) This invention, which is for improvements in or modifications of the invention the subject of British Patent No. 867,955, which is a patent of addition to British Patent No. 847,320, relates to television and like camera tubes generally known as image orthicon camera tubes. More specifically the invention relates to image orthicon camera tubes of the kind in which, in order to ensure that the electron beam scanning the target shall always be incident thereon at right angles thereto in all positions of cathode ray deflection, there is provided an electrostatic fieldterminating mesh which is mounted substantially parallel to and adjacent the target on the gun side thereof. This field-terminating mesh is for the purpose of correcting the electrostatic field in the neighbourhood of the target and increasing the strength of the decelerating field in front of the target. This reduces or eliminates distortion due to deflection of the ray by the electrostatic field resulting from the charge pattern stored on the target.

As is explained in the specifications of British Patent No. 867,955, and British Patent No. 847,320, operating defects occur with known image orthicon camera tubes of the kind referred to, because of secondary electron emission from the field-terminating mesh. One such defeet is that such secondary electrons released from the mesh as a result of bombardment by electrons of the ray on its way to the target cause an undesirably high DC. output. Also a distorting out-of-phase signal occurs due to secondary electrons released from the mesh returning to the electron multiplier section of the tube. Since these electrons start from the mesh with very low velocity, they will arrive at the input of the multiplier section a little later than (i.e. a little displaced in time with respect to) the main signal; in other words they cause an out-of-phase and therefore a distorting signal component to be included in the output from the tube. The parent invention, which seeks to eliminate or substantially reduce the above-mentioned defects, consists, in its broadest aspect, in providing means for establishing, in the neighbourhood of the entrance to the electron multiplier section of the tube, a potential surface which is negative with respect to the potential of the field-terminating mesh adjacent the target structure of the tube, whereby secondary electrons emitted by said mesh are substantially prevented from entering said multiplier section.

The invention of British Patent No. 867,955 provides a television camera tube arrangement comprising an image orthicon camera tube of the kind referred to wherein the field-terminating mesh adjacent the target structure is provided with its own independent connection lead, extending to a point outside the envelope of the tube and means for applying a potential, which is positive with respect to the potential of the first dynode of the electron multiplier section of the tube, to said lead.

According to this invention a television camera tube arrangement comprises an image orthicon tube of the kind referred to and means for applying to said fieldterminating mesh and to the normally provided internal wall coating on the inside of the tube envelope a common potential which is positive with respect to the potential of the first dynode of the electron multiplier section of the tube.

The field-terminating mesh may be connected internally of the tube to the wall coating thereof, in which case said common potential may be applied to a single external connection lead for the mesh and the coating, or alternatively the field-terminating mesh and the wall coating may be separate from one another and may have their own independent connection leads to which the common potential is applied.

Accordingly there is produced, between the mesh and the electron multiplier section of the tube, a negative potential gradient which prevents secondary electrons emitted from the mesh from reaching the said multiplier section.

In carrying out the present invention the common potential applied to the field-terminating mesh and to the wall coating of the tube is, of course, adjusted in potential to provide the required beam focus.

The invention is illustrated in the accompanying drawing which is a highly simplified schematic representation of a preferred embodiment of the invention. The drawing shows only those parts of the tube necessary to an understanding of this invention.

Referring to the drawing, an image orthicon tube of the kind referred to has an envelope 1 with a photo cathode 2 on the inside of the end wall thereof. 3 represents the usual internal coating of the inner wall of the tube. 4 is a field terminating mesh adjacent and parallel to the customary target structure, conventionally represented at 5, and is internally connected as shown to the internal wall coating 3. To the wall coating 3 is connected a connection lead 3a, shown for simplicity as going straight through the envelope wall, and to which in operation a suitable positive potential is applied. The electron gun is at the end of the neck of the tube, its cathode being given the reference 6. Arranged around the gun, as in the usual way, as a secondary electron multiplier section comprising a number of dynodes conventionally represented at 7 the first dynode being referenced 7a. The electron gun and the electron multiplier section are parts of a single mechanical unit or structure as in the customary way.

Means (not shown) are provided to apply a suitable potential, positive with respect to the first dynode 7a, to the internal coating 3 and the mesh 4 via the lead 3a. To give typical operating potentials by way of example only, the potential of the first dynode 7a might be about +289 volts and the potential of the internal coating 3 and the field-terminating mesh 4 about +320 volts, the latter voltage being chosen such as to provide the required beam focus and to provide between the fieldterminating mesh 4 and the dynode 7a a potential gradient such as to prevent secondary electrons given off from the mesh 4 from impinging on dynode 7a.

The tube 1 need not, of course, have the mesh 4 and the internal wall coating 3 internally connected to one another but each may have its own independent external connection lead, in which case the aforesaid common potential is applied to both connection leads.

I claim:

1. A television camera tube arrangement comprising an image orthicon tube having an electrostatic field terminating mesh, an internal wall coating on the inside of the tube, an electron multiplier section including at least one dynode, and means for applying to said field terminating mesh and to said internal wall coating a common potential which is positive with respect to the potential of said one dynode.

2. A television camera tube arrangement comprising an image orthicon camera tube having an envelope, an internal wall coating on the inside of said envelope, at photocathode on the inside of one end wall of said envetial which is positive With respect to the potential of the 10 dynode means of said electron multiplier section.

3. An arrangement as claimed in claim 1 wherein the field terminating mesh is connected internally of the tube a to the Wall coating thereof. 7 4. An arrangement as claimed in claim 1 wherein the field terminating mesh and the wall coating are separate from one another and have their own independent connection leads. 7

References Cited in the file of this patent I FOREIGN PATENTS Great Britain Sept. 7, 1960 Great Britain May 10, 1961 

1. A TELEVISION CAMERA TUBE ARRANGEMENT COMPRISING AN IMAGE ORTHICON TUBE HAVING AN ELECTROSTATIC FIELD TERMINATING MESH, AN INTERNAL WALL COATING ON THE INSIDE OF THE TUBE, AN ELECTRON MULTIPLIER SECTION INCLUDING AT LEAST ONE DYNODE, AND MEANS FOR APPLYING TO SAID FIELD TERMINATING MESH AND TO SAID INTERNAL WALL COATING A COMMON POTENTIAL WHICH IS POSITIVE WITH RESPECT TO THE POTENTIAL OF SAID ONE DYNODE. 